Air disk brake caliper pre-stressing method and pre-stressed caliper apparatus

ABSTRACT

A cast iron brake caliper with improved fatigue life, and a process and process equipment for pre-stressing a cast iron brake caliper to provide improved fatigue life, is provided. In the process, a load is applied to a cast iron caliper, where the load is high enough to locally yield and plastically deform the cast iron material, but not high enough to cause material failure, such as cracking. Upon load removal, residual compressive stresses in the cast iron caliper lower the mid-point of the stress range the plastically-deformed region of the caliper sees during in-service use, and thereby lowers the peak stress seen in this region, increasing fatigue life. The process permits a cast iron brake caliper to be designed to use less material and thus fit within constrained wheel rim envelopes, without the need to resort to high cost materials or other alternative design strategies.

BACKGROUND OF THE INVENTION

The present invention relates to brakes used on, for example, commercialtruck or trailer axles, and in particular to manufacture of brakecalipers used in such brakes.

Air disk brakes have been widely adopted in Europe, primarily as resultof their performance advantages over conventional drum brakes. Thesehigh performance brakes have not been widely adopted in U.S. commercialvehicles. One impediment to wide-scale adoption of disk brakes in theU.S. is the relatively small wheel rims used on U.S. commercialvehicles, as compared those used in Europe, and U.S. vehicle operators'general reluctance to incur the expense of shifting to larger wheelrims, at least in the absence of a regulatory requirement to do so.

The Society of Automotive Engineers is leading work to establish astandard commercial vehicle brake packaging recommendation forindustry-wide use, however, this effort has been ongoing for at leasteight years without issuance of a formal standard or recommendation, andnone is expected to be adopted as a regulatory requirement in the nearfuture (the issuance of a government requirement being seen as anecessity to get commercial vehicle owners to move away from the currentstandard U.S. wheels). Accordingly, in view of the practical realitiesof the current commercial vehicle wheel and brake environment, if airdisk brakes are to used on a large scale in the U.S. in the near future,air disk brake designs such as those used in Europe must be redesignedto fit within the limited clearance envelope of the existing standardU.S. wheel rim sizes.

Various approaches have been considered for such redesigns, such asusing materials other than the usual cast iron, using a smaller diameterbrake rotor, and designing the calipers to be thinner in the radialdirection to fit within a wheel rim. The common theme among thealternatives is attempting to decrease the radial height of the brakecaliper, typically by removing material from the portion of the caliperwhich bridges over the outer radius of the brake rotor (i.e., theportion of the caliper between the brake application side and thereaction side of the caliper). None of these solutions has yet toprovide a design without undesirable compromises, such as prohibitivecost (due to, for example, the use of higher strength, higher costmaterials) or insufficient strength and/or fatigue life due tounacceptably thin caliper sections.

Engineering calculations and testing have shown that when a brakecaliper is loaded during brake application, there are regions of veryhigh stress in and near the areas of the brake caliper which reach overthe outer radius of the brake disk. Calculations have demonstrated thatwhen the amount of cast iron in the cross-disk region of a brake caliperis reduced in order to obtain sufficient wheel rim envelope clearance,the stress levels in cast iron brake calipers manufactured usingconventional manufacturing methods are so high as to significantlyreduce the fatigue life of the caliper, to the point that adequatecaliper life cannot be assured.

Typical approaches to increase fatigue life include increasing theamount of material present in the highly-stressed region; modifying thegeometry of the component to further distribute and reduce stresses;moving to higher cost, higher strength materials such as steel alloys,and various surface treatments.

U.S. Pat. No. 5,841,033 shows a method of improving fatigue performancein steel components (a much more ductile material than cast iron, whichis brittle and unforgiving of excessive deflection). In this method, acompressive force is applied to specific points along the surface of thecomponents to pre-stress the component in localized areas. Thispre-stress is not applied over the entire surface of the components, orto inner regions.

U.S. Pat. No. 4,248,191 shows a pre-stressing method for use inminimizing the potential for cracks in engine cylinder heads. In thismethod, rings are added in a region between valve seats in the cylinderhead. These additional rings are intended to apply compressive forces inthe between-seat region, and thereby prevent the occurrence of tensilestresses in the cast iron cylinder head (i.e., if tensile forces areapplied to the cylinder head, the pre-stressing ring's compressiveforces are intended to be so high that the applied tensile forces neverovercome the compressive forces of the rings).

U.S. Pat. No. 5,193,375 shows a method to increase the life of a castiron brake drum by shot-peening the surface of the brake drum to relieveresidual stresses in the surface of the brake drum.

U.S. patent application No. US 2008/0081208 shows an element with atextured surface (i.e., a stamp) which is used to apply a surfacepre-stress to a component made of ductile materials such as a stainlessnickel-based alloy.

International Patent Publication WO 2004/078275 shows an aluminum orsteel golf club head with a pre-determined pre-stress applied to thesurface of the club head's striking face, so as to obtain a spring-likeeffect.

None of these references, however, teaches an approach to improvingfatigue life which is applicable to preventing failure of a brakecaliper, for example by generation of fatigue cracks, where thehighest-stressed material in the caliper is not necessarily located atthe surface of the caliper. Moreover, none of the references teach anyapproach which is compatible with cast iron brake calipers (theforegoing cast iron cylinder head reinforcing ring approach not beingrelevant to a cast iron brake caliper, as there is no room for theaddition of a reinforcing ring in the most highly stressed regions of abrake caliper).

SUMMARY OF THE INVENTION

The present invention provides a cast iron brake caliper with improvedfatigue life, and a process and process equipment for pre-stressing acast iron brake caliper to provide improved fatigue life.

It is known generally that the fatigue life of ductile materials may beincreased by shifting the range of operating stress on the material intoa region with lower peak stresses. FIG. 1 provides a exampleillustration of this effect in a modified Goodman diagram. This diagramshows the relationship between mean stress magnitude, stress rangeamplitude and the number of cycles to fatigue failure for an examplematerial. A first loading example A is repetitively loaded between aminimum stress level S_(min) (in this example, zero stress) and amaximum stress level S_(max). The midrange stress associated with thisstress range S_(r) (S_(r) being the difference between S_(max) andS_(min)) is used to locate a plot of the example A stress range on theordinate axis. In this example, plotting the example A stress range atits corresponding midrange stress places the example A minimum stresslevel on a line corresponding to a fatigue life of 10⁴ cycles.

A second loading example B shows the benefit of pre-stressing acomponent with compressive stress. In this example, the magnitude of thestress range Sr is the same as in example A. However, the application ofcompressive pre-stressing to the material has effectively shifted thestress range toward the compression region, resulting in a lowermidrange stress between the extremes of S_(max) and S_(min).Accordingly, when the example B is plotted on the modified Goodmandiagram at the lower midrange stress, the example B S_(min) valueintersects the fatigue life cycles lines at a fatigue life of 10⁶cycles. This is a substantial increase in fatigue life over the exampleA design, even though the example B material must endure loading overthe same large stress range as in example A.

FIGS. 2 a, 2 b and 2 c provide an example illustration of the stressdistribution in an object of simple geometry, to further illustrate theeffects of pre-stressing. FIG. 2 a illustrates a block 10 at threesuccessive times being loaded and unloaded in the elastic range.Initially the force applied at the center 20 of the left side of theblock 10 is at zero, and the corresponding stress across the thicknessof the block 10 at line A-A, shown below the block 10, is zero. As alater time, a force A in the elastic range is applied, and the force Ais resisted by two opposing points 30 on the right side of the block 10,resulting the block 10 being placed in bending. Thus, at point 20 theblock 10 is in compression as shown in the stress line below the block.Due to the bending load, the stress increases linearly to a tensionstate on the right side of the block 10. Finally, after the force A(which is still within the elastic range of the material) is removed,the stress across the cross-section of the block at line A-A returns tozero throughout the block 10.

FIG. 2 b illustrates the effect on block 10 from application of a forcehigh enough to pre-stress the block material. In the first illustrationin FIG. 2 b, the block 10 is at zero stress across its cross-section atline A-A, as was the case in FIG. 2 a. However, in this case a force Bis now applied, where the force B is higher than the elastic range forceA, such that force B causes plastic elongation of the block material atthe right side of block 10, as indicated in the stress line below theblock. The plastic elongation, i.e., a lateral shift of the materialresponse curve on a stress-strain diagram (a well-known phenomena whichoccurs at strains on the order of a few percent elongation in materialssuch as cast iron), is frequently manifested in the form of highlylocalized plastic deformation, but may also occur in a distributedmanner, for example, at a plurality of local points of stressconcentration.

As with FIG. 2 a, at the left side of the cross-section A-A the materialis in compression and the stress increases into the tensile range towardthe right side of the block 10. However, because the force B was highenough to plastically deform a portion of the block 10 (but not so highas to exceed the material's ultimate stress), the stress distributionbecomes non-linear once the yield stress is reached within the blockmaterial, and increases at a lower rate in the plastically-deformedregion at the right side of block 10. Then, once the force B is removedand the applied force again becomes zero, the material which is onlyelastically deformed (i.e., the material surrounding the plasticallydeformed region) attempts to return to its original shape. As a result,the return-to-original-position-seeking elastically deformed materialapplies a compressive pressure to the plastically deformed material.This effect is illustrated in the A-A cross-section stress distribution,which is no longer linear, but instead illustrates that the material atthe right side of the block 10 is under a compressive residual stress.

The benefit of the compressive residual stress realized in the FIG. 2 bloading is shown in FIG. 2 c. In this figure, the pre-stressed block 10initially has the residual stress distribution generated when the forceB was removed from the block 10 in FIG. 2 b. When a force in the elasticrange is now applied to block 10, the stress at the left side of blockat cross-section A-A is a compressive stress, as before. However, ratherthan the stress increasing linearly across the block as in the FIG. 2 anon-pre-stressed block, the stress from the applied force is summed withthe residual stress distribution across the block 10. As a result, thereis a new reduction of the stress level at the right side of the block10. After the elastic range force is removed, the residual stressdistribution returns to the pre-stressed distribution. Thus, whilepre-stressing of the block 10 does not result in reduction of themagnitude of the range of stress experienced at any given point withinblock 10, pre-stressing does result in a decrease in the maximum stressexperienced by the block, which in turn increases the number of cyclesthe block 10 may undergo before fatigue failure.

While pre-stressing of ductile materials is known, it has previouslybeen believed that pre-stressing was not a viable option for cast ironbrake calipers, in part because cast iron has only a fraction of theductility of materials such as steels, and in part because the mosthighly stressed portions of commercial vehicle brake calipers aretypically within the interior of the caliper structure, whereconventional surface deformation approaches to pre-stressing cannot beapplied and/or do not have any significant effect. In particular, castiron's lack of ductility is a problem because cast iron typically cannotendure significant deformation under tension before cracks form asplastic elongation limits are exceeded.

The present invention provides a solution to these and other problems ofthe prior art, by providing a cast iron brake caliper which is compactenough to fit within the wheel rim envelope of U.S. wheel rims, whilealso having significantly improved fatigue life. The present inventionalso provides a corresponding process for creating the caliper byapplying loads in a highly controlled manner to alter the internalstructure of the material of the caliper. The invention further meetsthe objective of providing a caliper-altering process which is bothcost-effective and simple for relatively unskilled labor and/or roboticsto perform in a production environment.

In one embodiment, a cast iron brake caliper is designed with a thinnerthan customary section which bridges the gap over a brake rotor betweenthe brake actuator and reaction sides of the caliper. In thepre-stressing process of the present invention, the cast iron caliper isthen subjected to a pre-stressing loading in a highly controlled mannerto alter the caliper's material structure in the most highly stressedregion(s) of the caliper. Preferably, the load is applied to the caliperin the same manner as during brake use in an installed vehicle brakeapplication, e.g., the portions of the caliper which are alongside abrake rotor during brake application are pushed apart in the same mannerwas when reaction forces are generated during brake application. Oncethe load is increased to a predetermined level, the load is maintainedfor a predetermined period, and then the load is decreased in acontrolled manner. The pre-stressing loading cycle may be repeated oneor more times.

The load applied in the inventive pre-stressing process is set highenough to cause elastic deformation in the majority of the brakecaliper, and further high enough to also cause a very limited amount ofplastic deformation to occur in the very highest-stressed regions of thecaliper (i.e., exceeding the yield limit in these highest-stressedregions, resulting in plastic elongation of the caliper material). Theload must be set high enough to cause the desired plastic elongation,but must also be controlled so that the load is neither too high norapplied for too long a period that the brake caliper structure begins tofail, for example by exceeding the plastic elongation limit (i.e., theultimate strength, which is the point at which plastic elongation ceasesand failure of the material begins)—a point which frequently (but notalways) is accompanied by the development of cracks in the material.

When the most highly-stressed portion of the brake caliper plasticallydeforms, the cast iron material in the deformed portion area iseffectively “stretched.” Then, as the load is removed, theelastically-deformed cast iron material surrounding theplastically-deformed material begins to return to its original shape. Asthe material which has been only elastically-deformed contracts back toits original shape, it applies compressive force to the “stretched”plastically-deformed portion. Once the pre-stressing load is completelyremoved, the stress in the brake caliper returns to zero, except in theplastically-deformed region, which is now (in the unloaded state)pre-stressed with a compressing stress from the surrounding material.

The development of compressive forces around the plastically deformedportion of the cast iron brake caliper effectively shifts the load rangeS_(r) of the plastically-deformed portion of the caliper to a lowermidrange stress on a Goodman diagram, similar to the shift in the FIG. 1diagram from example A to example B. Thus, as a result of the inventivepre-stressing process, the most highly-stressed portion of the cast ironbrake caliper sees lower peak stresses and lower midrange stresses, andthe fatigue life of the caliper is increased, in some instances by twoor more orders of magnitude. Moreover, because this process does notrely on surface treatments, it is capable of compressive pre-stressingof material wherever the region(s) of highest stress may be located,including within the internal structure of the cast iron brake caliper.

The inventive cast iron brake caliper pre-stressing process thus avoidsthe compromise solutions of the prior art. For example, there is no needfor the use of costly higher strength materials to provide the requiredfatigue service life (and the associated increases in manufacturingcosts and material processing difficulty associated with the use of suchmaterials, such as more expensive tooling required to deal with thehigher strength material), nor is there any need to use smaller brakerotors, thereby reducing the thermal capacity, braking force and pad androtor life performance of the disc brake.

The inventive pre-stressing process also provides additional benefits,including reducing the effect of stress risers, as well as reducing theeffects of sand casting defects on the strength of the caliper (those ofordinary skill in the art will recognize casting defects to befrequently occurring in series production sand casting, and that bycausing the material surrounding such defects to apply compressiveforces to the defects, the load range experienced by the defects isshifted downward, effectively reducing the effect of the defects onfatigue life). Accordingly, the present invention provides U.S.commercial vehicle operators with disk brakes which fit within thehighly constrained U.S. wheel rim envelope, yet are still light inweight, low in cost and have an improved service life.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a modified Goodman diagram, illustrating the effect ofshifting of a load range and midrange stress on the fatigue life of amaterial.

FIGS. 2 a, 2 b and 2 c illustrate the effects of pre-stressing on stressdistribution in a geometrically simple object.

FIGS. 3 a, 3 b and 3 c illustrate a brake caliper in accordance with anembodiment of the present invention and loads applied during brakeapplication.

FIG. 4 is a flow chart of a pre-stressing process in accordance with anembodiment of the present invention.

FIG. 5 is a flow chart of actions within one of the pre-stressapplication steps of FIG. 4.

FIGS. 6 a and 6 b are graphs of pre-stress loading and associatedprocess control limits for a pre-stressing process in accordance with anembodiment of the present invention.

FIG. 7 shows an oblique view of a pre-stressing apparatus for performingcast iron brake caliper pre-stressing in accordance with an embodimentof the present invention.

FIG. 8 shows an oblique view of the pre-stressing apparatus of FIG. 7with a cast iron brake caliper in position for application ofpre-stressing in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 3 a, 3 b and 3 c show an embodiment of a brake caliper 100 towhich a pre-stressing process is to be applied. Because the basic designand operation of such calipers is well known to those of ordinary skillin the art, the majority of the components of a disk brake have beenomitted from the figures for clarity.

FIG. 3 a is an oblique view from above of caliper 100, which includesbrake actuator side 110, reaction force side 120, and rotor bridgingportions 130 which join the brake actuator side 110 and reaction side120 over the outer radius of a brake rotor (not illustrated). As can beseen in FIG. 3 b, which is a sectioned view of the brake actuator side110 of the caliper 100 viewed from the reaction side, the bridgingportions 130 directly over the rotor are among the thinnest portions ofthe caliper. As a result, these thin sections 130 are among the mosthighly stressed portions of the caliper when a brake application forceis applied.

In this caliper embodiment, braking force is applied by a brake actuator(not illustrated) attached the rear face 140 of the brake actuator side110 of the caliper 100. The actuator applies brake application force toa lever (not illustrated) to advance brake pad application pistons (notillustrated) toward the brake rotor. As the lever transfers the brakingforce to the brake pad application pistons, a reaction force isgenerated, which is met by lever bearing seats 150 formed in therearward surface of the brake actuator side of the caliper. The reactionforce biases the brake actuator side 110, and thus the entire caliper100, laterally away from the brake rotor. The reaction force iscountered by the reaction side 120 of the caliper 100 when the brakepads on the reaction side of the caliper (not illustrated) come intocontact with the brake rotor on their front surfaces and press (viaintermediate components) against a pad abutment surface 160 in thereaction side 120 of the caliper.

As shown in FIG. 3 c, in this embodiment the generation of reactionforces as a result of brake application results in generating equal andopposite reaction force at lever bearing seats 150 of the brake actuatorside 110 and the pad abutment surface 160 of reaction side 120. Theseequal and opposite reaction forces are carried between the opposingsides of the caliper 100 through bridging portions 130. In addition tothe loads applied by the reaction forces, because the brake caliper 100is asymmetrical and the brake application reaction forces are applied incaliper regions radially well below the bridging portions 130, thesethin sections also must endure a high bending loading as the brakeactuator side 110 and reaction side 120 attempt to separate by rotatingabout the ends of the bridging portions 130. As will be evident to oneof ordinary skill in the art, due to the tensile reaction force loadingand the high bending loads, the bridging portions 130 are the regions ofthe caliper which typically see the highest loading during brakeapplication.

FIG. 4 is a flow chart of a pre-stressing method in accordance with anembodiment of the present invention. The method 200 starts with step210, at which a part to be pre-stressed, in this case a cast iron brakecaliper, is loaded into a pre-stressing fixture (the pre-stressingfixture is described further, below). The caliper may be partiallyassembled to facilitate the pre-stressing operation. For example, thecaliper may have installed load-bearing surfaces such as lever bearinginserts, needle bearings, a lubricated brake actuation lever, and aforce transfer component(s) (such as brake application pistons or asimple block) which spans a gap between the lever and the face of a loadtransfer component which transfers the pre-stressing load from the leverto the reaction side of the caliper.

At step 220 the caliper is identified, for example by a unique serialnumber read from a bar code or an RFI tag (radio frequencyidentification tag), and it is determined whether the caliper has beenpreviously subjected to pre-stressing. If the part has been previouslypre-stressed, the part is identified as such for subsequent marking andhandling as a re-worked part in step 225.

At step 230, the part is subjected to pre-stressing, in a mannerdiscussed further, below. In this embodiment, the pre-stressing force isapplied by loading the caliper in a manner similar to actual in-usebrake application loading. Specifically, a load application pushrod ofthe pre-stressing fixture applies the pre-stressing load to the brakeactuator lever of the caliper in accordance with a predetermined loadingschedule (in this embodiment, a linearly-increasing load, however,loading is not limited to a linear schedule). The pre-stressing load istransferred by the lever to the lever bearing surfaces on the brakeactuator side of the caliper, and through the gap-bridging components tothe pad abutment surface. During the pre-stressing load application,load, load rate, loading duration and part deflection are monitored todetermine whether the pre-stressing process has been properly applied(the monitoring discussed further, below).

Following the application of the pre-stressing load in step 230, in step240 a part which has been subjected to pre-stressing with all processparameters within specification is identified as a successfullypre-stressed part, appropriate part records are updated, and the partmay be marked to indicate that it has been successfully pre-stressed.The data collected may include serial number information for the part,as well as process data such as the maximum and minimum force during theloading cycle (or cycles, if more than one load cycle is practiced), themaximum part deflection during the cycle(s), and the initial and finalunloaded part deflection measurements. Upon completion of step 240, thepart is removed from the fixture in step 250 for further handling in theproduction process, and the process reaches its end 260.

The process of applying the pre-stressing load in step 230 involves anumber of actions, illustrated in the flow chart shown in FIG. 5. FIG. 5will be explained with the aid of FIGS. 6 a and 6 b, show correspondinggraphs of pre-stressing loading and process limits. At the beginning ofstep 230, in sub-step 300 the load application push rod begins to applythe pre-stressing load. The start of pre-stressing load application isshown in FIG. 6 a at time t1. Due to the nature of cast iron, thepre-stressing load 600 must be applied in a very carefully controlledmanner, and must be maintained within strict process limits 610, 620.Parameters which may be monitored include the required load, therequired rate at which the load is increasing, the required deflectionof the part in response to the applied load (an indication of whetherthe part is behaving in response to the load in the expected manner, or,for example, is deflecting an unexpectedly high amount, indicating thepart material has failed).

In the embodiment shown in FIG. 6 a, a cast iron brake caliper which isexpected to see an in-service load of 170 kN is initially pre-stressedby a load which ramps up from 0 to 255 kN (i.e., above the part'selastic range, but below the failure load) over a period of 5 seconds.One of ordinary skill will recognize that the pre-stressing load must beselected to suit the particular part being pre-stressed, depending onsuch variables as the cast iron metallurgy of the caliper, and thecaliper geometry (including, for example, the thickness and width of theportions between the brake actuator side and reaction side of thecaliper). For example, while the above-noted pre-stressing loading canbe expected to be appropriate for a large air disk brake caliper, suchas the Bendix Spicer Foundation Brake, LLC, Elyria, Ohio, Model ADB22X™22″ air disk brake caliper, lower pre-stressing loads can be expected tobe appropriate for smaller calipers. Thus, the peak pre-stressing loadfor typical commercial vehicle brake calipers may be expected to varybetween approximately 120 kN and 400 kN, depending on the design of theparticular caliper to be pre-stressed.

At sub-step 310, it is determined whether all of the process limits weremet during the load ramp-up in sub-step 300, including determiningwhether the load ramp rate and part deflection limits were exceeded. Ifthe process specifications were not met, then the part is marked asfailed in sub-step 320.

If the process limits during the loading ramp-up were determined to havebeen met in sub-step 310, then in sub-step, 330 the pre-stressingloading of 255 kN is maintained for a pre-determined period, in thisembodiment 5 seconds. At the end of the load maintaining sub-step 330,time t3, another determination is made in sub-step 340 as to whether allof the process limits were met during the load maintaining sub-step. Ifthe process specifications were not met, then the sub-process processproceeds to sub-step 320 for marking the part as failed.

If the process limits during the load maintaining sub-step were met,then between time t3 and time t4 the pre-stressing load is ramped downto zero in sub-step 350, in this embodiment over a period of 2seconds.At the end of the load ramping-down sub-step 350 (at time t4), it isagain determined in sub-step 360 whether all of the process limits weremet during the load ramping-down sub-step. If the process specificationswere not met, the part is marked as failed in sub-step 320.

As shown in FIG. 6 b, the pre-stressing process of the present inventionmay include more than one application of a pre-stressing load to thepart to achieve the desired alteration of the material structure of thepart. As shown in FIG. 6 b, the number of repetitions may be as few astwo, with a predetermined interval between the loadings (in thisembodiment, 2 seconds). Alternatively, more repetitions, such as fiveloading cycles, may be employed to obtain the desired fatigue lifeincrease. In the event multiple loading cycles are to be performed, theloading step 230 includes sub-step 370, in which it is determinedwhether the pres-stressing force application is to be repeated. If yes,the process returns to the sub-step 300 loading ramping-up process. Ifnot, the process proceeds to data recording step 240.

It will be readily apparent to one of ordinary skill in the art that theoperating parameters described in the above embodiments may be varied asnecessary to suit the part to be pre-stressed. For example, to obtain adesired amount of fatigue life increase for a particular application,the operator may vary the rate of loading increase and/or decrease, theduration of the period in which the load is maintained at a constantvalue, the length of time between loading cycles and/or the maximumpre-stressing load. Further, in addition to varying parameters forpre-stressing of different parts, or parameters may also be variedbetween different load cycles being applied to the same part.Improvement in fatigue life may be further enhanced by conducting thepre-stressing operation at different temperatures, and/or theapplication of a post-pre-stressing process heat treatment, for example,heating the pre-stressed cast iron brake caliper to 375° F. for a periodof 60 minutes. The length of time the pre-stressing load is maintainedmay also be set to remain below the amount of time required for thecaliper material to exhibit the creep phenomena and exceed thematerial's minimum % elongation (the creep phenomena being theelongation of a material over time as the material is held in a stressedstate).

The foregoing pre-stressing processing embodiment may be conducted on afixture which employs closed-loop process feedback control of theapplied pre-stressing force. The pre-stressing force may be applied in avariety of ways, for example, by a hydraulic actuator, as shown in theexample embodiment of such pre-stressing equipment illustrated in FIG.7. In this embodiment, the fixture 700 includes a fixture frame 710which provides support for a hydraulic actuator 720, caliper positioningblocks 730, and force transfer block 740. The force transfer block 740is supported on posts (here, bolts 745) on the caliper positioningblocks 720 at a pre-determined height above the hydraulic actuator 720.The force transfer block 740 is arranged to fill the internal spacebetween the opposing sides of the cast iron brake caliper (as shown inFIG. 8), and may be equipped with a load cell for monitoring and, ifdesired, controlling the load applied to the caliper during thepre-stressing process via closed loop feedback.

At the top end of the hydraulic actuator 720, a brake actuator pushrodtip 750 transfers the force generated by the hydraulic actuator to abrake actuation lever within the cast iron brake caliper (notillustrated in FIG. 7 for clarity). The fixture 700 also includes alower caliper deflection detection probe 760 and an upper caliperdeflection probe 770 which may be used to monitor the deflection of thecast iron brake caliper during the pre-stressing process, and furtherused to provide deflection feedback for closed loop control of the forceapplication during a pre-stressing process.

FIG. 8 shows an oblique view of the pre-stressing apparatus of FIG. 7with a cast iron brake caliper 780 in position for application ofpre-stressing. The brake actuator side 782 of the caliper is supportedon a support plate 790 carried by a caliper feeding machine (notillustrated), in this case a robotic equipment handling machine. Thebrake actuator side 782 is provided with a brake actuator lever (notillustrated within the caliper) located in the conventional manner, andassociated force transfer components (also not illustrated within thecaliper) which span the distance from the internal rear wall of thebrake actuator side 782 of the caliper to the lower face of the forcetransfer block 740. In a preferred embodiment, these associated forcetransfer components may be, for example, the caliper's usual in-servicebrake pad application mechanisms, which have been positioned in apartially-assembled brake caliper.

When the cast iron brake caliper 780 is in position for pre-stressing,the force transfer block 740 fits between the brake actuator side 782and the reaction side 784 of the caliper, and between the caliper bridgesections 786 connecting the opposing sides of the caliper. When thepre-stressing force (also referred to as the pre-stressing load) isapplied by the hydraulic actuator 720 through the brake actuator pushrodtip 750 to the caliper brake lever, the pre-stressing force is applied:(i) by the lever as a reaction force against the internal rear wall ofthe brake actuator side and as a brake actuation force through thelever's associated force transfer components against the lower face ofthe force transfer block 740, and (ii) via the top face of the forcetransfer block 740 to a reaction face of the reaction side 784 of thecaliper. The pre-stressing force therefore biases the brake actuatorside 782 away from the reaction side 784 during the pre-stressingprocess in a manner similar to the application of brake application andreaction forces during in-service brake application when a vehicle'sbrakes are applied.

The pre-stressing process in this embodiment is controlled by acontroller 790, schematically illustrated in FIG. 7. The controller 790receives sensor information via input 792, and outputs via output 794control signals which control the actuation of hydraulic actuator 720.For example, during the pre-stressing process, lower deflection probe760 (not shown in FIG. 8) is in contact with the brake actuator side782, and the upper deflection probe 770 is in contact with the reactionside 784 of the caliper. These sensors permit monitoring of the amountof deflection of the cast iron brake caliper resulting from the appliedpre-stressing load. Thus, in addition to controlling the application ofthe pre-stressing force to the cast iron brake caliper 780 in accordancewith a pre-programmed pre-stressing loading schedule (such as theloading schedule shown in FIG. 6 a), controller 790 may also applyclosed-loop feedback control based on sensor inputs to alter thepre-stressing loading, thereby ensuring that the pre-stressing processremains within its highly controlled limits. For example, theapplication of the pre-stressing load may be controlled to obtain thecaliper deflection to be a desired caliper deflection corresponding tothe specified pre-stressing loading schedule. Moreover, the amount ofcaliper deflection may be monitored such, for example, in the case ofthe use of the pre-stressing equipment embodiment of FIG. 7 on a BendixModel ADB22X™ 22″ air disk brake caliper, the deflection is controlledto be limited to 2.9 mm, in order to avoid undesired exceeding of theultimate strength of its cast iron material.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. For example, the inventionis not limited to a one-piece caliper, Rather, pre-stressing may beapplied to a multi-part caliper, such as a two-piece caliper built up bybolting together opposing halves of the caliper. Similarly, rather thanplacing an entire caliper housing in a pre-stressing machine andapplying the pre-stressing load to the entire caliper housing,pre-stressing may be applied to individual caliper sub-assemblies orparts which are later assembled into a complete caliper. For example, inthe case of a caliper which is built up from separate brake applicationside, reaction side and rotor-spanning (“arms”) parts, the applicationof pre-stressing only to the highly-stressed rotor-spanning parts priorto these parts' later incorporation into a complete caliper would bewithin the scope of the present invention. Because such modifications ofthe disclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A brake caliper pre-stressing apparatus,comprising: a fixture for receiving a cast iron brake caliper forpre-stressing; a pre-stressing force actuator adapted to apply apre-stressing load to the cast iron brake caliper; a force transfermodule for transferring a pre-stressing load from a brake actuator sideof the cast iron brake caliper to a reaction side of the cast iron brakecaliper; at least one sensor for detecting at least one pre-stressingcontrol parameter for feedback control of the pre-stressing loadapplication; a controller, wherein the controller is programmed toreceive detected pre-stressing control parameter information from the atleast one sensor and to control the pre-stressing force actuator tomaintain the pre-stressing force within a desired pre-stressing loadcontrol range, control application of the pre-stressing force inaccordance with a pre-determined pre-stressing loading schedule, wherethe pre-stressing loading schedule includes the steps of: applying thepre-stressing load to the cast iron brake caliper, wherein thepre-stressing load is high enough to cause localized plastic deformationin the cast iron brake caliper housing, and below a load high enough tocause failure of the cast iron brake caliper by crack initiation;maintaining the pre-stressing load a predetermined period sufficient topermit the localized plastic deformation to occur; and removing thepre-stressing load.
 2. The brake caliper pre-stressing apparatus ofclaim 1, wherein the at least one sensor includes at least one of acaliper deflection sensor and a pre-stressing load sensor.
 3. A computerproduct comprising a non-transitory machine-readable storage mediumhaving stored therein code segments for causing a brake caliperpre-stressing apparatus to pre-stress a cast iron brake caliper inaccordance with a pre-stressing loading schedule, the pre-stressingloading schedule including the acts of: applying a pre-stressing load tothe cast iron brake caliper, wherein the pre-stressing load is highenough to cause localized plastic deformation in the cast iron brakecaliper housing, and below a load high enough to cause failure of thecast iron brake caliper by crack initiation; maintaining thepre-stressing load a predetermined period sufficient to permit thelocalized plastic deformation to occur; and removing the pre-stressingload.
 4. The computer product of claim 3, wherein code segments causethe brake caliper pre-stressing apparatus to remove the pre-stressingload after the pre-determined period has expired.
 5. The computerproduct of claim 4, wherein code segments further include the acts of:determining whether a predetermined process specification is exceededfor at least one of applied load, load ramp-up rate, load duration, loadreduction rate and cast iron brake caliper housing deflection amount;and rejecting a caliper housing if at least one predetermined processspecification is exceeded.
 6. The computer product of claim 5, whereincode segments cause the brake caliper pre-stressing apparatus toincrease the pre-stressing load during the load applying in a controlledmanner in accordance with a predetermined pre-stressing loadingschedule, and decrease the pre-stressing load during the load removingin a controlled manner in accordance with a predetermined pre-stressingunloading schedule.
 7. The computer product of claim 5, wherein codesegments cause the brake caliper pre-stressing apparatus to perform theload applying more than one time to the same cast iron brake caliper. 8.The computer product of claim 6, wherein the predetermined pre-stressingloading schedule is a linear increase in load from an unloaded conditionto a predetermined maximum pre-stressing load, and the predeterminedpre-stressing unloading schedule is a linear decrease in load from thepredetermined maximum pre-stressing load to an unloaded condition. 9.The computer product of claim 8, wherein the applied pre-stressing loadis increased from 0 to between 120 kN and 400 kN.
 10. The computerproduct of claim 8, wherein the applied pre-stressing load is increasedfrom 0 to between 250 kN and 300 kN.
 11. The computer product of claim8, wherein the applied pre-stressing load is increased from 0 to between250 and 260 kN.
 12. A controller for controlling the operation of abrake caliper pre-stressing apparatus, the controller having storedtherein code segments for causing a brake caliper pre-stressingapparatus to pre-stress a cast iron brake caliper in accordance with apre-stressing loading schedule, the pre-stressing loading scheduleincluding the acts of: applying a pre-stressing load to the cast ironbrake caliper, wherein the pre-stressing load is high enough to causelocalized plastic deformation in the cast iron brake caliper housing,and below a load high enough to cause failure of the cast iron brakecaliper by crack initiation; maintaining the pre-stressing load apredetermined period sufficient to permit the localized plasticdeformation to occur; and removing the pre-stressing load.